Automatically configurable optical system for connecting customer premises networks to a switching center of a telecommuncation network

ABSTRACT

An automatically configurable optical connection system for connecting customer premises networks to a switching center of a telecommunication network includes an optical distribution network connecting all central office termination equipment optical senders to all customer premises network optical receivers and connecting all customer premises network optical senders to all central office termination equipment optical receivers. A control unit assigns a different wavelength to each customer premises network communicating at a given time, this wavelength representing an available central office termination equipment which is not the control central office termination equipment. To enable automatic system configuration each central office termination equipment sends cyclically an identification message and each inactive customer premises network listens out cyclically at each wavelength available to the central office termination equipments and memorizes the relationship between the wavelengths and the central office termination equipment identities, marking the control central office termination equipment wavelength. The system is for use in broadband telecommunication networks.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention concerns an automatically configurable optical system forconnecting customer premises networks to a switching center of atelecommunication network. An optical system of this kind comprises anoptical distribution network comprising optical fibers coupled toequipments called central office termination equipments in the switchingcenter and coupled to the customer premises networks. The switchingcenter and the telecommunication network are broadband systems. Coppercables are ill suited to transmission of data at high bit rates which iswhy optical fibers are used to connect the customer premises networks tothe switching center. To facilitate the adoption of a connection systemof this kind it is necessary to minimize the cost of implementing eachcustomer premises network and to ensure that services are extremelyreliable.

2. Description of the prior art

French patent application n° 91 12 264 describes an optical connectionsystem in which each central office termination equipment comprises anoptical sender and an optical receiver operating at the same fixedwavelength. Each central office termination equipment uses a differentwavelength. Each customer premises network comprises an optical senderand an optical receiver which can be tuned to different wavelengths. Awavelength is assigned to a customer premises network by a control unitin the switching center at the time each call is set up between thecustomer premises network and a central office termination equipment.The number of customer premises networks is very much larger than thenumber of central office termination equipments to which they areconnected because not all customer premises networks make or receivecalls simultaneously. Dynamic assignment of wavelengths to customerpremises networks means that the same wavelength can be used again fordifferent customer premises networks setting up non-simultaneous calls.

A group of customer premises networks is connected to a group of centraloffice termination equipments by a distribution network which broadcastsan optical signal sent by any sender of the connection system to eachoptical receiver of the connection system. A switching center maycomprise a plurality of identical connection systems each comprising agroup of central office termination equipments connected to a group ofcustomer premises networks by independent broadcast networks so that acommon set of wavelengths can be re-used several times.

A control central office termination equipment in each group of centraloffice termination equipments sets up calls. When a customer premisesnetwork goes active, it sends a message using the control central officetermination equipment wavelength. The control central office terminationequipment sends the message to the control means of the switchingcenter. The control means tell the control central office terminationequipment the identity of a central office termination equipmentavailable to set up a call. The control central office terminationequipment sends a message to the customer premises network using thecontrol central office termination equipment wavelength. This messagetells the customer premises network the identity of the availablecentral office termination equipment. The customer premises networkdeduces from this the wavelength at which it must send and receive touse the central office termination equipment assigned to the call. Thiswavelength is assigned to this customer premises network for the entireduration of this call, and for this duration only.

In order to function a customer premises network needs to know thecontrol central office termination equipment wavelength and therelationship between the identities indicated in the messages sent bythe control central office termination equipment and the wavelengths. Inthis prior art system a customer premises network can know thisinformation only if it is provided when it is installed. For example, itis feasible to program this information manually into the customerpremises network by means of small switches or by plugging in aread-only memory module. This type of method has the drawback ofrequiring a certain amount of work each time that a customer premisesnetwork is installed and lacks flexibility because it is not possible toreplace a faulty central office termination equipment with anothercentral office termination equipment having another wavelength withoutdoing the same work over at all customer premises networks. Inparticular, it is not possible to assign the control central officetermination equipment function to any of the central office terminationequipments installed in the switching center. If a central officetermination equipment fails it must be replaced with another centraloffice termination equipment having the same wavelength. Consequently,either a large number of central office termination equipments havingpredetermined wavelengths must be kept in reserve or a single centraloffice termination equipment must be kept in reserve having a wavelengthwhich is set when it is put into service.

An object of the invention is to propose a connection system that isconfigurable and reconfigurable automatically to avoid the need for workon each customer premises network when the latter is installed and tominimize the disturbance to services in the case of a fault in a centraloffice termination equipment or in the distribution network.

SUMMARY OF THE INVENTION

The invention consists in an automatically configurable opticalconnection system for connecting customer premises networks to aswitching center of a telecommunication network, said switching centercomprising a connection network, a connection network control unit andcentral office termination equipments, each central office terminationequipment comprising an optical sender and an optical receiver tuned tothe same wavelength, different for each central office terminationequipment, each customer premises network comprising an optical senderand an optical receiver tuned to the same wavelength which is variable,the wavelength of the receiver being set to the wavelength of a controlcentral office termination equipment when the customer premises networkis not communicating, the system comprising:

an optical distribution network connecting all central officetermination equipment optical senders to all customer premises networkoptical receivers and connecting all customer premises network opticalsenders to all central office termination equipment optical receivers,and

a control unit for assigning a different wavelength to each customerpremises network communicating at a given time, said wavelengthrepresenting an available central office termination equipment which isnot the control central office termination equipment,

in which system:

each central office termination equipment comprises means for assigningto said central office termination equipment an identity and means forcommanding the optical sender of said central office terminationequipment to send cyclically an identification message containing theidentity of said central office termination equipment and indicating ifthe control central office termination equipment function is assigned toit or not,

each customer premises network comprises means for, when said customerpremises network is inactive:

tuning its receiver successively to each of the wavelengths available tothe central office termination equipments,

waiting at each wavelength for a predetermined time to receive anidentification message,

memorizing the central office termination equipment identity for eachwavelength on which the customer premises network has received anidentification message,

marking the identity of each control central office terminationequipment among the memorized identities, and

tuning its receiver to the wavelength of a control central officetermination equipment.

With this system, when each customer premises network is put intoservice it automatically determines the system configuration, i.e. itautomatically establishes a relationship between the various wavelengthsthat it may receive and the various central office termination equipmentidentities that may be mentioned in messages sent by the control centraloffice termination equipment to set up calls. This system enables thecustomer premises network to distinguish automatically the wavelength ofthe control central office termination equipment, on which each customerpremises network must "listen out" when it is inactive (on standby).Because this information can be updated cyclically, the customerpremises networks can be advised very quickly of changes in thefunctions assigned to the various central office termination equipmentsto remedy failure of one central office termination equipment or a partof the optical distribution network. These function changes may bemanual or the result of an automatic test and reconfiguration procedure.

According to another feature of the system in accordance with theinvention, the switching center comprises means for:

assigning the control central office termination equipment function toat least one central office termination equipment operating correctly,

monitoring the operation of each central office termination equipment,and

assigning the control central office termination equipment function to adifferent central office termination equipment operating correctly ifthe previous control central office termination equipment fails.

This system enables the set of central office termination equipments tobe reconfigured automatically should any of them fail because theswitching center includes means for assigning the control central officetermination equipment function to any other central office terminationequipment and because each customer premises network can take note ofthis change of control central office termination equipmentautomatically.

According to another feature the optical distribution network of thesystem in accordance with the invention comprises:

at least a first stage of optical couplers connected to said centraloffice termination equipments,

at least a second stage of optical couplers connected to said customerpremises networks, and

fibers connecting said first stage couplers to said second stagecouplers, at least two optical fibers linking each second stage couplerto at least one first stage coupler, using different paths, so thatfailure of either optical fiber does not prevent any customer premisesnetwork from being connected to at least one central office terminationequipment.

This system is particularly reliable because it enables at least sometelecommunication services accessible to a subscriber to be maintainedif a failure occurs between the two stages of the optical distributionnetwork, i.e. on the optical cables connecting the switching center to acentral office termination equipment serving a small group of customerpremises networks.

The invention will be better understood and other details of theinvention will emerge from the following description and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of a connection system inaccordance with the invention.

FIG. 2 shows timing diagrams illustrating the operation of thisembodiment.

FIG. 3 is a functional block diagram of this embodiment.

FIG. 4 shows timing diagrams illustrating the operation of thisembodiment.

FIG. 5 shows the main stages of call set-up in this embodiment.

FIG. 6 is the block diagram of one embodiment of a customer premisesnetwork.

FIG. 7 is a block diagram of one embodiment of the central officetermination equipment adapted to be connected to the customer premisesnetwork shown in FIG. 5.

FIGS. 8 and 9 are timing diagrams showing the exchange of data betweenthe customer premises network and the central office terminationequipment.

FIG. 10 is a block diagram of one embodiment of a broadcast networkadapted to limit the effects of a transmission cable being cut in oneembodiment of the system in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one embodiment of a connection system in accordance withthe invention comprising a group of 256 customer premises networks CPN0through CPN255 connected by an optical distribution network DN to aswitching center EX which is part of a broadband asynchronous transfermode telecommunication network. The number of customer premises networks(256) is chosen purely by way of example. The total number may behigher. The upper limit is set by the capabilities of the opticaltransmission technology. Each customer premises network CPN0 throughCPN255 has an optical input-output connected by a single optical fiberF0 through F255 to the distribution network DN which in the blockdiagram is shown as a single optical fiber. The switching center EXcomprises a connection network CN and 32 central office terminationequipments ETE0 through ETE31. Each central office termination equipmenthas an optical input-output connected by a single optical fiber F'0through F'31 to the distribution network DN, an electrical input and anelectrical output respectively connected to an electrical output and anelectrical input of the connection network CN. The switching center EXfurther comprises a control unit CU connected by a link B to eachcentral office termination equipment ETE0 through ETE31 and connected toan input-output of the connection network CN.

The control central office termination equipment function is assigned toone of the 32 central office termination equipments (central officetermination equipment ETE0, for example) by the control unit CU. Thisassignment can be reconsidered if the control unit CU detects that thechosen central office termination equipment is no longer operatingcorrectly.

The control unit CU monitors the operation of each central officetermination equipment ETE0 through ETE31 continuously, verifying thateach call assigned to a central office termination equipment isprocessed correctly and that the control central office terminationequipment is capable of sending and receiving messages. If a centraloffice termination equipment fails it is no longer used and itsfunctions are assigned to another central office termination equipment.This other central office termination equipment may be a standby centraloffice termination equipment which is installed but which is not usuallyused. A standby central office termination equipment may be provided foreach group of 32 central office termination equipments, for example. Ifthe control central office termination equipment fails the fault isparticularly serious because it is no longer possible to set up a callbetween a customer premises network and any other central officetermination equipment.

However, the control unit CU can rectify this fault immediately byassigning the control central office termination equipment function toanother central office termination equipment that is operatingcorrectly: all the central office termination equipments are identicalapart from their different, fixed wavelengths.

They comprise all the control means they need to implement either thecontrol central office termination equipment function or the callcentral office termination equipment function, as assigned by thecontrol unit. When the control central office termination equipment isreplaced with another, all customer premises networks must change thewavelength on which they "listen out" when on standby to receive callsand on which they request call set-up. Similarly, when a customerpremises network has just been created it begins by determining theconfiguration of the connection system, i.e. the relationship betweenthe 32 wavelengths that its receiver may receive and the numbersidentifying the central office termination equipments with which it cancommunicate.

FIG. 2 shows the operation of the embodiment described above in respectof determining the system configuration. The first three timing diagramsshow the cells sent by the respective central office terminationequipments ETE0, ETE1, ETE2 taken by way of purely illustrative example.The final timing diagram shows the value of the wavelength to which thereceiver of a customer premises network on standby (customer premisesnetwork CPN10, for example) is tuned. To determine the wavelength usedby the central office termination equipments the receiver scans theentire range of wavelengths available to the central office terminationequipments (Lmin through Lmax) continuously. Each central officetermination equipment sends cells continuously. The cells are empty ifthe central office termination equipment has no data to send. Eachcentral office termination equipment sends cyclically a cell containingan identification message: MI0, MI1, MI2 respectively for central officetermination equipments ETE0, ETE1, ETE2.

The receiver starts at time t0 by tuning to the wavelength Lmin. At timet1 it receives a signal at wavelength L0. It tunes its filter to thewavelength L0 of the received signal and waits to detect anidentification message at this wavelength. At time t2 it receives anidentification message MI0 telling it that central office terminationequipment ETE0 is sending and that this is a control central officetermination equipment (in this example). The customer premises networkmemorizes the wavelength L0 by memorizing a filter tuning parametervalue, the identity of the central office termination equipment ETE0 anda bit indicating that it is a control central office terminationequipment. The receiver then resumes scanning the range of wavelengths.

At time t3 scanning is again halted because the receiver has detected asignal at wavelength L1. It waits for an identification message. Itreceives a message MI1 at time t4. It memorizes the wavelength L1, thecentral office termination equipment identity ETE1 and a bit indicatingthat it is not the control central office termination equipment. It thenresumes scanning the range of wavelengths.

At time t5 the receiver stops scanning again because it receives asignal at wavelength L2. It waits for an identification message at thiswavelength. It receives an identification message MI2 at time t6. Itmemorizes the wavelength L2, the central office termination equipmentidentity ETE2 and a bit indicating that it is not a control centraloffice termination equipment. It then resumes scanning the range ofwavelengths.

This scanning continues up to the maximal wavelength Lmax after whichthe receiver returns to standby at the control central officetermination equipment wavelength L0.

Scanning is resumed subsequently, in a cyclic manner. It may beinterrupted if the customer premises network is used, in order to set upa call, for example. The identification message waiting time is limitedto a predetermined value: if the receiver does not detect anyidentification message within this time, it resumes scanning the rangeof wavelengths.

The number of customer premises networks connected to a group of centraloffice termination equipments depends on the load on the group ofcentral office termination equipments that the customer premisesnetworks represent. Generally speaking, customer premises networks arenot active simultaneously, i.e. they are not all communicating at thesame time. Consequently, the number of central office terminationequipments may be less than the number of customer premises networks. Ina preferred embodiment, time-division multiplexing in the connectionsystem in accordance with the invention enables the number of customerpremises networks to be increased provided that each one has a bandwidthrequirement which is substantially lower than the bandwidth provided bya central office termination equipment.

When a customer premises network is inactive its receiver is on standbyat the control central office termination equipment wavelength. When acall is set up between a customer premises network and a central officetermination equipment the optical sender and the optical receiver of thecustomer premises network are set to the wavelength of this centraloffice termination equipment. For more efficient use of central officetermination equipment bandwidth the same wavelength may be assigned to aplurality of customer premises networks or a plurality of simultaneouscalls, by assigning it to each call during time slots separate from thetime slots of other calls.

In FIG. 1, for example, the customer premises networks CPN0, CPN10,CPN20 are communicating with the central office termination equipmentETE31 at the same wavelength L31. The data exchanged between thecustomer premises networks and the central office termination equipmentsis all in the form of cells with a standardized asynchronous transfermode format. To prevent mutual interference between the data sent by thethree customer premises networks CPN0, CPN10, CPN20, the cellscontaining the data are sent during non-contiguous time slots, which maybe repeated periodically to constitute a variety of time-divisionmultiplexing superposed on the spectral multiplexing carried out by theoptical distribution network.

FIG. 3 is a functional block diagram of this embodiment of a connectionsystem in accordance with the invention. It shows the new functions ascompared with those already provided in the system described in Frenchpatent application n° 91 12264. All the customer premises networks CPN0through CPN255 may comprise identical functional units.

The customer premises network CPN0, for example, comprises: one or moreterminal equipments TE such as telephones, videophones, fax machines,computers, TV sets, etc, a subscriber digital network termination NT1 asdefined in CCITT Recommendation I.413 and a remote user equipment RUE.The remote user equipment RUE implements the known functions describedin the patent application n° 91 12264 to assign a wavelength to thecustomer premises network CPN0 plus additional functions fortime-division multiplexing consisting essentially in commanding thesending of data only during a time slot defined by the control unit CUand functions for memorizing the central office termination equipmentidentities and marking the control central office termination equipment.

All the central office termination equipments ETE0 through ETE31comprise identical functional units. The central office terminationequipment ETE0, for example, comprises: a central office terminationequipment "nucleus" ETEN implementing the known subscriber connectionfunctions and a local user equipment LUE which implements the functionsdescribed in the French patent application n° 91 12264 and essentiallyconsisting in sending and receiving at a fixed wavelength and convertingoptical signals into electrical signals fed to the connection networkand reciprocally converting electrical signals into optical signals fedto the distribution network DN, plus additional functions required bytime-division multiplexing including regulating the throughput of datareconstituted by the central office termination equipment to theconnection network CN and controlling time-division multiplexing andfunctions for sending cyclically a central office termination equipmentidentification message.

The throughput is regulated to prevent throughput peaks which compromiseproper functioning of the connection network. Time-division multiplexingof data entails grouping cells in time slots respective to the variouscustomer premises networks which send at the same wavelength. As thecells may be sent at different rates by each customer premises networkand as time-division multiplexing sends the cells from each customerpremises network in bursts, the central office termination equipmentETE0 receives cells at a rate which varies according to the time slot.In the absence of throughput regulation the throughput to the connectionnetwork CN would thus be subject to significant variations.

The remote user equipments RUE of the set of customer premises networksCPN0 through CPN255 and the local user equipments LUE of the set ofcentral office termination equipments ETE0 through ETE31 constitute adistributed concentrator DC.

There are various ways for a call to be set up at the request of one ofthe customer premises networks CPN0 through CPN255. One way is for thecontrol unit CU to have the control central office termination equipmentsend cyclically a series of interrogation messages respectivelyaddressed to all inactive customer premises networks. A customerpremises network requiring to set up a call responds immediately bysending a response message to the control central office terminationequipment on its wavelength. An alternative method is for each customerpremises network to be authorized to send a call request message to thecontrol central office termination equipment on the latter's wavelengthat periodic short time intervals. For this method to be used eachcustomer premises network must be advised beforehand of: the time, theduration and the period of the time interval in which it is authorizedto send. These parameters are supplied to the customer premises networkat the end of each call, for example. The duration of the time intervalmay be very short because the call request message sent to the centraloffice termination equipment can be restricted to a very small amount ofdata.

The requested call is then set up using a variety of time-divisionmultiplexing with other calls already set up to optimize use of centraloffice termination equipment bandwidth.

FIG. 4 shows eight timing diagrams illustrating the time-divisionmultiplexing of data sent by two customer premises networks CPN1 andCPN2 which have set up two independent calls on the same wavelengthusing the same central office termination equipment ETEi. When the twocalls are set up the control central office termination equipment ETEcsends a control message CM1 to the customer premises network CPN1 andthen, after a time interval dt1, sends a control message CM2 to thecustomer premises network CPN2. These messages are shown on the firstand fifth lines of FIG. 3, respectively. The second and sixth lines ofFIG. 3 respectively show the reception of the messages CM1 and CM2 bythe customer premises networks CPN1 and CPN2. The third and seventhlines of FIG. 3 respectively show the sending of data by the customerpremises networks CPN1 and CPN2. The fourth and eighth lines of FIG. 3respectively show reception by the central office termination equipmentETE of the information sent by the customer premises networks CPN1 andCPN2.

The message CM1 is received by the customer premises network CPN1 aftera time-delay TD1. The message CM2 is received by the customer premisesnetwork CPN2 after a time-delay TD2 which is different than TD1 becausethe two customer premises networks are at different distances from theswitching center EX. The message CM1 tells the customer premises networkCPN1 that it is authorized to send after a delay time for sending DTS1starting with receipt of the message CM1 and for a duration D1. Themessage CM2 tells the customer premises network CPN2 that it isauthorized to send after a delay time for sending DTS2 starting fromreceipt of the message CM2 and for a duration D2.

The control unit CU of the switching center EX knows the time-delays TD1and TD2 either because these were measured when the customer premisesnetworks CPN1 and CPN2 were created or because they are measuredperiodically by means of a test message. The control unit CU calculatesthe delay times DTS1 and DTS2 on the basis of the time-delays TD1 andTD2 and the duration dt1 of the time interval between sending of themessages CM1 and CM2. For example, the control unit CU notes first thatthe time-delay TD1 is shorter than the time-delay TD2 and so decides tochoose a delay time DTS1 shorter than the delay time DTS2. The delaytime DTS1 is chosen at random except that its value must be greater thanthe time needed for a customer premises network to receive the messageCM1 completely and interpret it. The duration D1 is chosen according tothe bandwidth required for the customer premises network CPN1. In thisexample it is assumed to be twice that required for the customerpremises network CPN2 which is why the duration D1 is twice the sendduration D2.

The control unit CU deduces the time t1 at which reception by thecentral office termination equipment ETEi of the data sent by thecustomer premises network CPN1 ends. It adds to time t1 a duration dt2as a safety margin, yielding the time t2 at which reception by thecentral office termination equipment ETEi of the data sent by thecustomer premises network CPN2 should begin. The control unit CU thendeduces the delay time DTS2 knowing the time-delay TD2 and the durationdt1 of the time interval between sending of the message CM1 and sendingof the message CM2.

There is strictly speaking no synchronization between the clocks of thevarious customer premises networks and those of the central officetermination equipments. These clocks are merely plesiochronous. Eachcustomer premises network uses the time it receives a control message asa reference to start measuring the duration of the delay time assignedto it.

In another preferred embodiment ETEi sends a synchronization messageperiodically to all CPNj. Each CPNj synchronizes its own clock to thetime it receives the synchronization message. The send delay times DTSjare measured from reception of the sychronization message. In thisalternative embodiment the same messages CM1 and CM2 are used to conveyrespective data DTS1, D1 and DTS2, D2; however, the send times aredetermined from receipt of the synchronization message. This enables themessages CM1 and CM2 to be sent without consideration as to the timeinterval dt1.

Taking the time at which the synchronization message is sent as the timeorigin, the data sent by a customer premises network CPNj is received inETEi between times:

    2TDj+DTSj

and

    2TDj+DTSj+Dj

The synchronization message may also be used for other purposes. It mayinclude the identifier of the central office termination equipment ETEiand an indication as to whether it is the control central officetermination equipment or not. It may command a particular CPNj torespond with a predetermined message telling it DTSj. The time at whichthis message is received at ETEi enables TDj to be measured.

In one embodiment the central office termination equipment must send acontrol message CM for each sending by a customer premises network. Inanother embodiment a control message authorizes a customer premisesnetwork to send data periodically, the period being also indicated inthe message or determined once and for all for a given type customerpremises network. This periodic sending may be stopped by a decision ofthe customer premises network or by a decision of the control unit ofthe switching center, the latter sending a particular control message tothe customer premises network concerned via the central officetermination equipment used for the current call.

FIG. 5 shows the exchange of data between a customer premises networkCPN, a control central office termination equipment ETEc, a centraloffice termination equipment ETEi in communication with the customerpremises network CPN and the control unit CU of the switching center.The control central office termination equipment ETEc is identical tothe other central office termination equipments but is used by thecustomer premises networks as their interlocuter for setting up calls.Each customer premises network has a table showing the relationshipbetween the respective wavelengths and the respective identities of thevarious central office termination equipments of the connection system,including the wavelength of the control central office terminationequipment ETEc. When a customer premises network is inactive its opticalreceiver is tuned systematically to the control central officetermination equipment wavelength. There are two possible situations:

The customer premises network CPN requests call set-up: it sends amessage to the control central office termination equipment ETEc on thelatter's wavelength either after an interrogation message sentcyclically by the control central office termination equipment or in atime interval reserved to the customer premises network, i.e. by one ofthe two methods previously described. The control central officetermination equipment ETEc receives this message and sends it to thecontrol unit CU of the switching center. The control unit CU sends amessage to the customer premises network CPN via the control centraloffice termination equipment ETEc. This message includes the identity ofan available central office termination equipment ETEi, the control unitCU knowing the status of each central office termination equipment byvirtue of its connection over the B link. The customer premises networkCPN then tunes its optical sender and its optical receiver to thewavelength of the central office termination equipment ETEi assigned toit.

Call set-up is requested by a network subscriber who wishes to call acustomer premises network served by the optical connection system. Ifthe customer premises network is not already in communication thecontrol unit CU commands the control central office terminationequipment ETEc to send the identity of an available central officetermination equipment ETEi to the called customer premises network CPN.The customer premises network receives this message and tunes itsoptical sender and its optical receiver to the wavelength of the centraloffice termination equipment ETEi assigned to it. If the customerpremises network CPN is already in communication with another centraloffice termination equipment the control unit CU uses this other centraloffice termination equipment to dialog with the customer premisesnetwork.

Calls are of three types: broadcast from a central office terminationequipment to a plurality of customer premises networks with notransmission in the opposite direction, broadcast from a central officetermination equipment to a plurality of customer premises networks withtransmission in the reverse direction, and point-to-point transmissionfrom one central office termination equipment to one customer premisesnetwork and vice versa.

The connection system is particularly well suited to broadcast typecalls because any optical signal sent by a central office terminationequipment is systematically routed to all customer premises networksconnected to the same broadcast network. The optical receivers of thecustomer premises networks receiving a broadcast merely need to be tunedto the wavelength of the central office termination equipmentbroadcasting the call. On the other hand, in this type of connectionsystem point-to-point calls must be encyphered to prevent customerpremises networks discovering the content of calls that are notaddressed to them. Irrespective of the call type, the data exchangedbetween the customer premises networks and the central officetermination equipments is in the form of cells to a standardizedasynchronous format. This format comprises a virtual circuit identifierfield and a virtual circuit group identifier field which togetheridentify each call. The equipment ETEN of the central office terminationequipment ETEi assigns a value to these fields for each call and theremote user equipment RUE of the customer premises network CPNrecognizes this value for each call.

Encyphering may be carried out as follows: a coding law is chosen by thelocal user equipment LUE of the customer premises network CPN and isthen sent in a protected form to the local user equipment LUE of thecentral office termination equipment ETEi assigned to the call. Codingand decoding are thus carried out using the same law in the centraloffice termination equipment and the customer premises networkcommunicating with each other.

Another encyphering method is to allocate a fixed encyphering law foreach customer premises network and to tell each connection systemcentral office termination equipment the encyphering law of eachcustomer premises network of the system. This simplifies the exchange ofsignalling messages between the customer premises network and thecentral office termination equipment but means that each customerpremises network must be initialized with a predetermined encypheringlaw.

Consider the main message sent by the local user equipment LUE of acentral office termination equipment to the remote user equipment RUE ofone or more customer premises networks CPN:

a message, as already indicated, is used by the control central officetermination equipment ETEc to tell each customer premises network thetime and the duration of the time interval in which it is authorized tosend data,

a message is sent by each central office termination equipmentperiodically or pseudo-periodically to indicate the identity of thecentral office termination equipment in order to maintain in the remoteuser equipment RUE of each customer premises network CPN an up-to-datetable showing the relationship between each wavelength that the customerpremises network may receive and the identities of the central officetermination equipments which are actually accessible to that customerpremises network, and

a message is sent by the central office termination equipment (thecontrol central office termination equipment ETEc for example) tocommand a customer premises network to discard the wavelength to whichit is tuned and to tune to another wavelength as indicated by themessage.

Some of these message are sent in clear and can be received andprocessed by all the remote user equipments RUE of all the customerpremises networks.

Consider the messages sent from a customer premises network CPN to acentral office termination equipment. These messages are usuallyconventional signalling messages generated by a terminal T1 but thereare at least two specific messages generated by the remote userequipment RUE:

a message to indicate the encyphering law to be used, when setting up acall; this message is sent to the central office termination equipmentETEi used for a call after the control central office terminationequipment ETEc has commanded the customer premises network to tune to awavelength of a central office termination equipment ETEi other than thecontrol central office termination equipment ETEc, and

a message sent to the control central office termination equipment ETEcto indicate that the customer premises network requires to access abroadcast service whose identity is specified in the message.

FIG. 6 shows the setting up of a point-to-point call or a point-to-pointcall combined with broadcasting. Call set-up begins with a requestmessage 100 sent by the terminal equipment TE and passed on to thecontrol central office termination equipment ETEc by the remote userequipment RUE on the wavelength Lc of the control central officetermination equipment. The latter sends the message (step 101) to thecontrol unit CU of the switching center EX. The latter responds to thecontrol central office termination equipment ETEc (step 102) with amessage giving the identity of another central office terminationequipment ETEi selected from the central office termination equipmentsavailable at the time in question, the delay time for sending DTS whichdefines the time at which sending is authorized and the duration D forwhich sending is authorized. The control central office terminationequipment ETEc then sends a message to the customer premises network(step 103). The latter tunes its optical receiver and its optical senderto the wavelength Li of the central office termination equipment ETEi.

This data is stored in the remote user equipment RUE and the remainderof the message is sent to the terminal equipment TE. The terminalequipment responds with an acknowledge message (step 104). The remoteuser equipment RUE adds to this acknowledge message an indicationdefining the encyphering law to be used for this point-to-point call.The message is received by the central office termination equipment ETEiand then sent to the control unit CU (step 105). The control unit CUauthorizes the central office termination equipment ETEi to set up thecall, carrying out the encyphering defined by this law (step 106). Thecentral office termination equipment ETEi sends the encyphered data tothe customer premises network CPN (step 107). The remote user equipmentRUE decyphers the data and sends it to the terminal equipment TE (step108). The customer premises network CPN recognizes that the message isaddressed to it by recognizing the value of the virtual circuitidentifier and virtual circuit group identifier fields which containparticular values stored in tables at the terminal equipment TE.

The terminal equipment TE sends data to the addressee subscriber andsignalling data (step 109). The signalling data is sent to the controlunit CU by the central office termination equipment ETEi (step 110). Thedata addressed to the called telecommunication network subscriber issent to that subscriber via the connection network CN.

FIGS. 6 and 7 are respectively block diagrams of one embodiment of partof the customer premises network CPNj and one embodiment of the localuser equipment LUEi of a central office termination equipment ETEiproviding two-way communication on a single optical fiber and requiringonly one tunable laser. The tunable laser is in the central officetermination equipment ETEi and provides a carrier for both transmissiondirections. The customer premises network CPNj comprises:

a terminal 1 such as a telephone (intermediate equipments between theterminal 1 and the other equipments listed below are not shown),

a device 2 for decyphering data addressed to terminal 1 (optional),

a device 13 for encyphering data from the terminal 1,

a photodetector 3 such as an avalanche photodiode,

two couplers 4 and 7,

two optical isolators 6 and 9,

an optical filter 5 tunable by an electrical signal supplied to acontrol input,

a control circuit 10,

an optical amplifier 11 whose gain is controlled by an electrical signalapplied to a control input, and

an electronic buffer memory 12.

The customer premises network CPNj is connected to the distributionnetwork DN by a single optical fiber 8 used for transmission in bothdirections. The optical signal arriving on the fiber 8 passes throughthe coupler 7, the isolator 6, the filter 5 and the coupler 4 and thenis converted into an electrical signal by the photodetector 3. Part ofthe optical signal is sampled by the coupler 4, amplified by theamplifier 11, passes through the isolator 9 and is injected back intothe fiber 8 by the coupler 7 to be conveyed towards the switchingcenter. The photodetector 3 has an electrical output connected to aninput of the decyphering circuit 2 and to an input of the controlcircuit 10. The encyphering circuit 13 has an output connected to aninput of the buffer memory 12. A control input of the buffer memory 12is connected to an output of the control circuit 10. The control inputof the optical amplifier 11 is connected to an output of the buffermemory 12. The control input of the filter 5 is connected to an outputof the control circuit 10.

As the signalling data is not encyphered it is sampled at the output ofthe photodetector 3 so that it can be supplied to the control circuit10. The other data is encyphered, however, and requires decyphering inthe device 2. The data sent by the terminal 1 may be encyphered by theencyphering device 13 and then stored in the buffer memory 12 pendingauthorization for it to be sent. The control circuit 10 controls thewriting and reading of this data in the buffer memory 12. The data readin the buffer memory 12 is used to modulate the gain of the amplifier 11in a binary fashion. The amplifier 11 modulates the amplitude of thefraction of the optical signal obtained by the coupler 4 and this avoidsthe need for a tunable laser in each customer premises network.

Another function of the control circuit 10 is to tune the filter 5. Whenthe customer premises network CPNi is created, and periodicallythereafter while it is inactive:

it tunes the filter 5 successively to each of the 32 wavelengthsavailable to the central office termination equipments,

at each wavelength it waits for a predetermined time to receive anidentification message,

if it receives an identification message during this time it memorizesthe central office termination equipment identity contained in theidentification message with a flag bit indicating if it is a controlcentral office termination equipment or not,

if it does not receive any identification message it memorizes a binaryword indicating that no central office termination equipment isoperating at this wavelength, and

when all 32 wavelengths have been scanned, it looks up in memory thewavelength of a control central office termination equipment and tunesthe filter 5 to this wavelength, on which it remains on standby for apredetermined time at the end of which it starts scanning the 32wavelengths again.

When a call is being set up, at the request of a network subscriber, thecontrol circuit 10 receives on the control central office terminationequipment wavelength a message telling it the identity of a centraloffice termination equipment to which the call is assigned. It thentunes the filter 5 to this wavelength and acknowledges reception, asdescribed already with reference to FIG. 5.

Each inactive central office termination equipment sends cellscontinuously. If it has no data to send it sends empty cells having apredetermined format. The control circuit 10 includes a clock slaved bya known method to the clock rate of the cells it receives, whether emptyor not. The presence of empty cells at a particular wavelength tells thecustomer premises networks that a central office termination equipmentis present on this wavelength. The control circuit 10 further comprisesa known type control device which slaves the filter at all times to thewavelength of the received cells.

FIG. 7 is a block diagram of one embodiment of the local user equipmentLUEi of a central office termination equipment ETEi which comprises:

a coupler 21,

an optical isolator 22,

a laser 23 tuned to a fixed wavelength which is different for eachcentral office termination equipment ETE,

an encyphering device 24,

a decyphering device 25 (optional),

a photodetector 26 such as an avalanche photodiode, for example,

an optical filter 27 tuned to a fixed wavelength which is different foreach central office termination equipment ETE,

a control circuit 28 connected by the link B to the control unit CU ofthe switching center, and

a buffer memory 29.

The central office termination equipment ETEi is connected to thedistribution network DN by a single optical fiber 20 used fortransmission in both directions. The optical fiber 20 is connected to aport of the coupler 21. Two other ports of the coupler 21 arerespectively connected to an output of the optical isolator 22 and to aninput of the filter 27. The coupler 21 sends to the fiber 20 an opticalsignal supplied by the isolator 22 and addressed to the distributionnetwork DN. The coupler 21 samples an optical signal from thedistribution network DN and applies it to the input of the filter 27which has an output connected to an input of the photodetector 26.

An electrical output of the photodetector 26 is connected to an input ofthe control circuit 28 and to an input of the decyphering circuit 25. Anoutput of the circuit 25 is connected to an input of the buffer memory29. An output of the buffer memory 29 is connected to an input of theconnection network CN via the equipment ETENi. A control input of thebuffer memory 29 is connected to an output of the control circuit 28.

One output of the connection network CN is connected via the equipmentETENi to an input of the encyphering circuit 24. An output of thecircuit 24 is connected to an amplitude control input of the laser 23.An output of the control circuit 28 is connected to another amplitudecontrol input of the laser 23.

FIGS. 8 and 9 illustrate the operation of the customer premises networkCPNi and the central office termination equipment ETEj when they are incommunication. FIG. 8 shows the amplitude Ar as a function of time t ofthe optical signal at the output of the filter 5, i.e. the opticalsignal propagating from the central office termination equipment to thecustomer premises network. FIG. 8 shows the amplitude At as a functionof time t of the optical signal at the output of the amplifier 11 whichis used as a modulator, i.e. the amplitude of the optical signalpropagating from the customer premises network to the central officetermination equipment. The reference symbols Ar and At are repeated inFIG. 6 to show where these amplitudes exist.

The amplitude Ar varies between a high level Ph and a low level Pb whichis a long way a way from the zero amplitude in order to supply to thecustomer premises network an optical signal of sufficient energy toenable the transmission of a return signal. The logic levels aredetected at the output of the photodetector 3 by comparing the amplitudeof the electrical signal supplied by the photodetector 3 with athreshold value sd2 equal to half the sum of the values Ph and Pb.

The amplifier modulates the amplitude of the signal supplied by thecoupler 4 and comprising a fixed fraction (one half, for example) of thesignal applied to the input of the coupler 4 and shown in FIG. 8. Theamplifier modulates this signal between a minimal value P0 which isclose to zero and a maximal value which is equal to the amplitude Pb orPh, depending on the signal supplied to the amplifier at the time inquestion. In the central office termination equipment ETEj the coupler21 extracts this signal and applies it to the photodetector 26. A logiclevel at the output of the detector 26 is detected by comparing theamplitude of the electrical signal supplied by the detector 26 with athreshold value sd1 equal to half the sum of the values P0 and Pb.

The bit rate need not be the same in both directions. It may be 600Mbit/s in the direction from the central office termination equipment tothe customer premises network and 150 Mbit/s in the direction from thecustomer premises network to the central office termination equipment,for example.

In the customer premises network CPNi the buffer memory 12 stores datato be sent until sending is authorized. The data is supplied at aconstant rate by the terminal 1 in the form of cells having thestandardized asynchronous transfer mode format. The function of thebuffer memory 29 in the central office termination equipment ETEj is toregularize the throughput of this data before it is sent to theconnection network CN because each input of an asynchronous transfermode connection network is designed for a particular maximal bit rate.This maximal rate must not be exceeded. Also, fewer cells are lost inthe connection network if the network input bit rates are regular.

The control circuit includes, for example, switches for assigning anidentity number to the central office termination equipment. It furthercomprises a processor which has a program for:

Sending a central office termination equipment identification messagecyclically using the laser 23. This message contains the central officetermination equipment identity number and a bit indicating whether it isa control central office termination equipment or not.

Sending the messages previously mentioned for implementing the controlcentral office termination equipment function or the call central officetermination equipment function, as appropriate.

FIG. 10 is the block diagram of a preferred embodiment of thedistribution network DN for connecting a switching center EX' tocustomer premises networks CPN0' through CPN255'. This embodiment hasthe advantage of maintaining service even if failure of the distributionnetwork DN cuts off half the transmission channels in the distributionnetwork DN.

The switching center EX' comprises, for example, 32 central officetermination equipments ETE0' through ETE31' and two optical couplers 40and 41 each having 16 ports respectively connected to 16 outputs of thecentral office termination equipments ETE0' through ETE31' and fourports respectively connected to four optical fibers of the distributionnetwork DN. Four fibers of which two are connected to the coupler 40 andtwo are connected to the coupler 41 constitute a cable 35. The otherfour fibers connected to these couplers constitute a cable 36 routeddifferently than the cable 35. If either cable is damaged accidentallythe other cable is not affected.

Near the customer premises networks CPN1' through CPN255' thedistribution network DN further comprises four optical couplers 31through 34 each having two ports connected to two fibers of thedistribution network DN and 64 ports respectively connected to 64customer premises networks CPN1' through CPN255'. The coupler 34 has twoports respectively connected to two ports of the coupler 41 by twofibers a and b of the respective cables 36 and 35, for example. If cable35 is broken the coupler 34 is still connected to the coupler 41 byfiber a of cable 36. Consequently, the 64 customer premises networksconnected to the coupler 34 are still able to access 16 of the centraloffice termination equipments ETE0' through ETE31' so that aconsiderable part of the service can still be provided. If somesubscribers are unable to reach the control central office terminationequipment the control central office termination equipment function canbe assigned to a second central office termination equipment.

Many variants of this embodiment will suggest themselves to the manskilled in the art, retaining the general principle of transmission bytwo independent cables 35 and 36 to the same coupler. The number ofoptical couplers used and the number of ports which each optical couplercomprises depend on energy considerations: optical receiver sensitivity,optical sender power output, and the presence or absence of opticalamplifiers in the distribution network DN.

The invention is not limited to the customer premises network andcentral office termination equipment embodiments shown in FIGS. 5 and 6.Any bidirectional transmission system using the same wavelength and thesame optical fiber may be adapted to implement the invention. Inparticular, modulation processes other than amplitude modulation may beused.

Each central office termination equipment has a fixed wavelength, butthe central office termination equipments may comprise a tunable opticalsender to render the central office termination equipments readilyinterchangeable. The optical sender is then tuned to a fixed wavelengthwhen the system is installed.

There is claimed:
 1. An automatically configurable optical connectionsystem, comprising:a switching center comprising: a connection network,a plurality of central office termination equipments, each of thecentral office termination equipments coupled to the connection network,and a connection network control unit coupled to the connection networkand coupled to each of the central office termination equipments, eachof the central office termination equipments operating at a fixedwavelength which is different for each of the central office terminationequipments, each of the central office termination equipments includingan optical sender and an optical receiver which are tuned to the fixedwavelength, at least one of the central office termination equipmentsbeing assigned by the control unit to be a control central officetermination equipment; a plurality of customer premises networks, eachof the customer premises networks comprising an optical sender and anoptical receiver tuned to an assigned wavelength which is variable, theassigned wavelength being set to the fixed wavelength of the controlcentral office termination equipment when the customer premises networkis not communicating; and an optical distribution network connecting theoptical sender of each of the central office termination equipments tothe optical receiver of each of the customer premises networks, andconnecting the optical sender of each of the customer premises networksto the optical receiver of each of the central office terminationequipments, the control unit assigning different wavelengths to each ofthe customer premises networks communicating at a given time, thedifferent wavelengths corresponding to available central officetermination equipments which are not the control central officetermination equipment, each of the central office termination equipmentshaving a unique identity indicator and further comprising means forcommanding the optical sender to send cyclically an identificationmessage to the optical distribution network, the message containing theunique identity indicator of the central office termination equipmentand indicating whether the central office termination equipment has beenassigned to be the control central office termination equipment, each ofthe customer premises network further comprising means for, when thecustomer premises network is inactive: tuning the receiver of thecustomer premises network successively to the wavelength of each of thecentral office termination equipments; waiting at each wavelength for apredetermined period of time to receive the identification message;memorizing a central office termination equipment identity for eachwavelength on which the customer premises network has received theidentification message; marking the identity of the control centraloffice termination equipment among the memorized identities; and tuningthe receiver of the customer premises network to the wavelength of thecontrol central office termination equipment.
 2. System according toclaim 1, wherein said switching center further comprises meansfor:assigning a control central office termination equipment function toat least one of the central office termination equipments that isoperating correctly, monitoring operation of each central officetermination equipment, and assigning the control central officetermination equipment function to a different central office terminationequipment that is operating correctly if a previous control centraloffice termination equipment fails.
 3. System according to claim 1wherein said optical distribution network comprises:at least a firststage of optical couplers connected to said central office terminationequipments, at least a second stage of optical couplers connected tosaid customer premises networks, and fibers connecting said first stagecouplers to said second stage couplers, at least two optical fiberslinking each second stage coupler to at least one first stage coupler,using different paths, so that failure of either optical fiber does notprevent any customer premises network from being connected to at leastone central office termination equipment.